1. Field of the Invention
The present invention generally relates to a wireless LAN (Local Area Network) system. More particularly, the present invention is concerned with an access point apparatus for measuring or determining the current location or position of a mobile terminal in the wireless LAN system by making use of a radio signal. Incidentally, the access point apparatus may also be referred to as the access point station or base station in more general terms.
2. Description of the Related Art
For better understanding of the concept underlying the present invention, description will firstly be made of background techniques of the invention. As a method of measuring or determining the current location of a terminal with the aid of a radio signal used in the wireless LAN system, there can be mentioned a method which is disclosed in A. Ogino et al's article “INTEGRATED WIRELESS LAN ACCESS SYSTEM, (1) STUDY ON LOCATION SYSTEM” in “The Collection of Lecture Drafts in the General Convention of The Institute of Electronics, Information and Communication Engineers in Japan (2003)”, B-5-203, p.662. In the following, description will be made of the configuration and operation of the wireless LAN access point apparatus (base station) disclosed in the Ogino et al reference.
FIG. 9 of the accompanying drawings is a schematic diagram which shows, by way of example, a configuration of a wireless LAN system equipped with a terminal locating facility (i.e., function for determining a location or position of a terminal). A plurality of access point apparatuses or base stations (e.g. three access point apparatuses 1, 2 and 3 in the case of the LAN system illustrated in FIG. 9) are connected to a server 6 by way of a network 5 constituted by a public network and a mobile communication network. The terminal denoted by reference numeral 4 performs transaction of information or data with the server 6 through the medium of these access point apparatuses or base stations.
A processing flow for determining the location of the terminal 4 in the LAN system shown in FIG. 9 will be described below. The terminal 4 transmits or sends a radio signal. The access point apparatuses or base stations 1, 2 and 3 receive the radio signal to execute the signal processing which is required for determining the location of the terminal. In addition, each of the access point apparatuses 1 to 3 informs the server 6 of the result of the above-mentioned signal processing as executed via the network 5. On the basis of the signal processing result received from the individual access point apparatuses or stations, the server 6 arithmetically determines the location of the terminal 4.
FIG. 10 of the accompanying drawings is a block diagram showing generally a structure of a hitherto known or conventional access point apparatus for realizing the processing flow mentioned above. Referring to the figure, a signal receiving unit 101 is designed to perform a reception processing at a high/intermediate frequency corresponding to the radio signal received via an antenna 100, a demodulation processing of a base-band signal and an AD conversion (Analog to Digital conversion) to thereby generate a received signal.
On the other hand, a received signal memory control module 106 is so designed or programmed as to perform an access control for storing the received signal generated by the signal receiving unit 101 in a received signal memory 102 in response to a capture start command messaged from a terminal location determining signal processing module 103 via a signal line 104. In that case, the received signal memory control module 106 holds as a capture start timing the timing which is indicated by a clock 107 when storage of the received signal in the received signal memory 102 is started. In this conjunction, the clock 107 may be constituted, for example, by a counter which operates under the timing of a clock signal used in the access point apparatus.
Next, referring to FIGS. 11 and 12 of the accompanying drawings, description will turn to an access control method carried out by the received signal memory control module 106 in the conventional access point apparatus or station. It is assumed that the storage of the received signal is started at a timing Tp0 shown in FIG. 11. In that case, the received signal memory control module 106 firstly stores the timing Tp0 as the capture start timing. In addition, the received signal memory control module 106 stores the received signal at the timing Tp0 in the received signal memory 102 at an address Ap0, as shown in FIG. 12. In this conjunction, the address Ap0 may be a predetermined address or alternatively it may be designated by the terminal location determining signal processing module 103 upon issuance of the capture start command from the terminal location determining signal processing module 103. In succession, the received signal memory control module 106 carries out the access control such that the received signal is stored sequentially in the received signal memory 102 at the addresses Ap0+1, Ap0+2, . . . in this order, as is illustrated in FIG. 12. When the received signal has been stored up to the address Ap1 in the received signal memory 102, the received signal memory control module 106 terminates or ends the process of storing the received signal in the received signal memory 102. Incidentally, the address Ap1 may be a predetermined address or alternatively it may be designated by the terminal location determining signal processing module 103 when the received signal capture start command is issued by the terminal location determining signal processing module 103.
Upon completion of the storage of the received signal in the received signal memory 102, the received signal memory control module 106 sends a received signal capture end message and the capture start timing to the terminal location determining signal processing module 103 via a signal line 105.
Upon reception of the capture end message mentioned above, the terminal location determining signal processing module 103 executes the signal processing required for arithmetically determining the location of the mobile terminal on the basis of the received signal stored in the received signal memory and the capture start timing mentioned above, the result of the signal processing being then messaged or transferred to the server 6 shown in FIG. 9. As the signal processing required for the arithmetic determination of the location or position of the mobile terminal, there can be mentioned a processing for arithmetically determining the reception timing of the signal sent from the terminal, a processing for arithmetically determining a delay profile for the determination of the reception timing, etc. In this conjunction, it should be added that a matched filter, for example, may be employed for the arithmetic determination of the delay profile. Further, for arithmetically deriving the reception timing of the received signal from the delay profile as obtained, a method disclosed in e.g. JP-A-2002-14152 may be adopted. Alternatively, the received signal stored in the received signal memory 102 may be intactly transferred to the server 6 without executing any particular processing on the received signal stored in the received signal memory 102.
The server 6 then arithmetically determines the location or position of the terminal on the basis of the results of the signal processings sent from the individual access point apparatuses (base stations) 1, 2 and 3 by resorting to, for example, a trilateration method described, for example, in the Ogino et al reference cited hereinbefore.